(1) Field of the Invention
The present invention relates to a secondary battery having a high energy density, a low self-discharge rate and a long cycle life.
(2) Description of the Related Art
A secondary battery in which lithium metal, one of the alkali metals, is used for the negative electrode is well known, and the secondary battery of this type is outlined, for example, by M. Hughes et al. in Journal of Power Sources, 12, pages 83-144 (1984). With regard to the problem of the lithium metal negative electrode, this reference points out that, since lithium is overactive, it reacts with an electrolyte, especially a solvent, to form an insulating film and cause growth of a dendrite during charging or discharging, and thus the charging or discharging efficiency is reduced or a short circuit occurs between positive and negative electrodes. To solve this problem, an attempt was made to use a lithium/aluminum alloy for the negative electrode, to reduce the activity of the negative electrode and control the reaction with the electrolyte, but, it is known that, if the charging/discharging cycle is repeated, the alloy becomes powdery and is easily broken. The results of an examination of the electrode characteristics of alloys of lithium with other metals are revealed by A. N. Dey in J. Electrochem. Soc., 118, No. 10, pages 1547-1549 (1971). This reference shows the results of comparative tests of changes of Li.sup.+ plating potentials and plating current efficiencies, made with respect to alloys of lithium with one metal selected from Sn, Pb, Al, Au, Pt, Zn, Cd, Ag and Mg and combinations of lithium with metals which are difficult to form into an alloy such as Ti, Cu, and Ni.
Furthermore, Japanese Examined Patent Publication No. 48-24302 proposes a secondary battery comprising lithium as the negative electrode active substance and a nickel halide as the positive electrode active substance, in which, to improve the charging efficiency of the lithium electrode and realize a good maintenability, a powdery mixture containing powdery lithium, which is bonded, together with a permanently conductive substance such as nickel powder or degassed granular carbon, to a grid support structure through a binder such as polyethylene or carboxymethyl cellulose, is used instead of a lithium foil.
Japanese Unexamined Patent Publication No. 59-14264 proposes a double-charging type lithium negative electrode especially suitable for a battery operated by an ionic polymer, i.e., a flexible composite anode comprising a lithium-containing finely divided alloy or intermetallic compound such as lithium/aluminum, lithium/silicon, lithium/antimony, lithium/bismuth or lithium/boron, or finely divided lithium, a plastic or elastomer type polymeric substance having an ionic conductivity, and a finely divided carbon additive such as carbon black or graphite.
Japanese Unexamined Patent Publication No. 59-132576 proposes a lithium negative electrode comprising a layer of a conductive polymer capable of forming a lithium ion-inserted compound, which is arranged on a lithium surface confronting a positive electrode active substance of a lithium secondary battery; Japanese Unexamined Patent Publication No. 59-157973 proposes an electrode for a secondary battery, which comprises a carbon fiber layer formed on the surface of an alkali metal; and Japanese Examined Patent Publication No. 59-186274 proposes a secondary battery comprising a negative electrode formed by using a material capable of absorbing an alkali metal ion at the time of charging, and releasing the alkali metal ion at the time of discharging, i.e., a fusible alloy, in which the alkali metal and the negative electrode material are compressed and integrated to improve the cycle life.
Japanese Unexamined Patent Publication No. 60-262351 proposes the use of a composite material of a lithium alloy and a conductive organic polymer to obtain a negative electrode in which a reduction of the performance does not occur even at a high lithium utilization ratio, and Japanese Unexamined Patent Publication No. 61-245474 proposes a non-aqueous secondary battery composed of a polymer having a main chain having a conjugated structure and a substance capable of forming an alloy with an alkali metal or a substance in which an alkali metal ion can be inserted.
Further, a non-aqueous secondary battery comprising a conductive polymer film formed on the counter-electrode-confronting surface of a fusible alloy negative electrode is disclosed in Japanese Unexamined Patent Publication No. 62-140358, and a non-aqueous electrolyte secondary battery comprising a negative electrode having lithium adsorbed in a negative electrode-constituting body composed of a mixture of a metal capable of forming an alloy with lithium or a lithium alloy powder with powdery graphite is disclosed in Japanese Unexamined Patent Publication No. 62-226536.
Many reports have been made on a alkali metal battery comprising a positive electrode composed of an inorganic oxide or inorganic calcogenide capable of absorbing and releasing an alkali metal ion upon charging and discharging, and an electrochemical intercalation of sodium to sodium-cobalt oxide is reported by Calude Delmas et al. in the Journal of Solid State Chemistry, 6, pages 532-537 (1973) or by Claude Delmas et al. in Solid State Ionics, 3-4, pages 165-169 (1981). In these references concerning sodium-cobalt oxide, it is stated that, when various oxides having different crystal structures are electrochemically oxidized or reduced, the crystal structures are changed by the quantity of the sodium ion according to the degree of oxidation, and that among these crystal structures, the P2 phase in which the oxygen arrangement is prismatic does not undergo a structural charge over a broad range of the sodium ion quantity. Accordingly, it is suggested that if this phase is used for an electrode, the theoretical energy density is highest.
An example in which a battery is fabricated by using sodium-cobalt oxide for the positive electrode is disclosed in Japanese Unexamined Patent Publication No. 61-245474, and in this example, a mixture of a powdery NaPb.sub.0.26 Sn.sub.0.74 alloy, poly-p-phenylene and a polypropylene binder is used for the negative electrode.
Various electrolytes for the alkali metal battery have been proposed. As the solvent, propylene carbonate (PC), dimethylsulfoxide (DMSO), dimethylformamide (DMF), tetrahydrofuran (THF), 2-methyltetrahydrofuran (MTHF) and 1,3-dioxolan (DOL), as proposed in the Journal of Power Source, 12, pages 83-144 (1984), are often used. Organic solvents and electrolytic substances are described in detail on pages 30 through 44 of Basic Electrochemical Measurement Methods compiled by the Association of Electrochemistry. A battery in which a mixture of a polyethylene glycol dialkyl ether and propylene carbonate is used as the solvent for improving storage characteristics of a lithium battery is disclosed in Japanese Unexamined Patent Publication No. 62-29070. Before this patent publication, in the Journal of Power Sources, 12, pages 53-59 (1984), Shinichi Tobishima and Akihiko Yamaki reported a mixed solvent having a good conductivity and utilizing a solvation effect, which comprises propylene carbonate and diglyme, triglyme or tetraglyme.
Nevertheless, practical batteries comparable to existing lead-acid batteries or nickel-cadmium batteries have not been developed from the foregoing proposals, and the problems of the batteries proposed in the foregoing literature and patent references inhibit a practical utilization thereof. The problems, etc., are summarized and shown by Junichi Yamamoto in Electrochemistry, 56, No. 1, pages 5-8 (1988) and Zenichiro Takehara in Chemical Industries, January 1988, pages 52-56.
A room temperature-operating secondary battery using an alkali metal for the negative electrode has the problems described above, and none of the batteries of this type is as marketable as a general-purpose secondary battery, although a lithium type secondary battery having a very small capacity (1 mAh to 3 mAh) has been marketed in very small quantities. Moli Energy Limited Co., Canada, marketed a secondary battery having a relatively large capacity (higher than 600 mAh) using MoS.sub.2 for the positive electrode, an Li foil for the negative electrode and an LiAsF.sub.6 type electrolyte, but this secondary battery was inferior to a nickel-cadmium battery of the same shape in reversability of the charging-discharging cycle, high-speed charging and discharging characteristic, and overdischarge characteristic, although the energy density was improved. Namely, this secondary battery has no general-purpose utility.
The causes of the difficulty of practical application are now under investigation, to clarify the problems of the conventional techniques. The problems involved in the use of an alkali metal element, especially lithium as the negative electrode, are due to the high activity of lithium per se. Namely, lithium has a very high reactivity with other substances, and the lithium surface always reacts with the electrolyte and impurities contained therein during storage, charging, and discharging of the battery. Accordingly, the electrode surface is partially or entirely covered with an insulating film acting as a resistance to the battery reaction, and the charging and discharging efficiencies are reduced. Moreover, during charging a dentrite inevitably grows to form a short circuit to the counterelectrode, and the life of the battery is shortened.
To overcome the above-mentioned problems arising when lithium element is used as the negative electrode, the lithium surface must be covered with a uniform ionic conductive film and the charging current density must be maintained at a low level at which the formation of a dendrite can be controlled. But, even if such means is adopted and the battery is operated under such conditions, every time charging-discharging is repeated, a new lithium surface is formed, and since this reaction does not participate in the charging and discharging, lithium is wastefully consumed, which is one reason why the life of the battery cannot be prolonged.
On the other hand, where sodium element is used as the negative electrode, since the ionization potential is higher by about 0.3 V than that of lithium, the reaction with the electrolyte is controlled to some extent, but the problems are not substantially different from those arising when lithium element is used. Moreover, since the reactivity with water or other is higher than that of lithium, handling is difficult and a practical utilization thereof is impossible.
Accordingly, if an alkali metal element is used directly as the negative electrode, a secondary battery having a good performance cannot be formed, and thus the use of an alkali metal alloy has been attempted as an excellent method for moderating the high activity of an alkali metal and appropriately controlling the battery reaction.
For example, as described in detail in J. Electrochem, Soc., 118, No. 10, pages 1547-1549 (1971), Journal of Power Sources, 12, pages 83-144 (1984), B. M. L. Rao, R. W. Francis and H. A. Christopher, J. Electrochem. Soc., 124, No. 10, pages 1490-1492, and J. R. Owen and W. C. Maskell, Solid State Ionics, 13, pages 329-334 (1984), the use of alloys of lithium with aluminum, tin, lead, magnesium, or zinc has been proposed. Among these alloys, a lithium/aluminum alloy is considered most excellent because the diffusion speed of lithium is highest in aluminum. The most important reason for using the alloy instead of the alkali metal element is that, as pointed out hereinbefore, the activity of the alkali metal is reduced to control the reaction with the solvent and impurities and the formation of a dentrite by this reaction is prevented. If the alloy is used, the electrodeposition potential of the alkali metal can be shifted to the noble side and the underpotential of electrodeposition can be utilized. For example, an alloy comprising lithium and aluminum at an atomic ratio of 1/1 has a potential nobler by 0.3 to 0.4 V than that of a lithium element. Accordingly, the reaction with a solvent which is readily reduced and decomposed or a substance with readily reacts with the lithium metal element can be controlled. In general, the formation of a dendrite is conspicuous when the current density at electrodeposition is high or the potential is low. This problem is substantially solved if the alloy is used.
Nevertheless, the problems arising when lithium is used as the electrode are not completely solved even if the alloy is used; some problems remain and new problems arise. Namely, even if the potential is shifted by 0.3 to 0.4 V to the noble side by using the alloy, the reaction with the solvent or impurities is not substantially inhibited.
For example, propylene carbonate, which is an organic solvent used relatively frequently, is thermodynamically decomposed even at a potential nobler by at least 0.4 V than that of lithium, and it is known that in the case of an ether type solvent, the reaction of which with lithium is relatively mild, impurities contained therein cannot be completely removed and the solvent reacts gradually with lithium because of the instability thereof. Moreover, the alloy is different from the lithium element in that the alloy electrode is thinned as charging-discharging is repeated, and finally, it becomes impossible to maintain the electrode form and the electrode is broken to shorten the life of the battery. Moreover, since the alloy is used, to obtain an electric capacity density comparable to that attained by lithium element, the electrode must hold an excessive additional portion, i.e., a mating metal to be alloyed with lithium, and the utilization of lithium in the alloy at every charging-discharging cycle must be considerably increased. An alkali metal secondary battery comprising a lithium alloy or sodium alloy alone as the electrode active substance and having an increased alkali metal utilization ratio, a large capacity, and a good cycle life has not been practically marketed.
As the means for improving the utilization ratio of the lithium negative electrode, that is, the electric capacity density, a process in which the surface area of the electrode is increased by mixing the lithium element or a lithium alloy with a carbon material, as described hereinbefore, is disclosed in Japanese Examined Patent Publication No. 48-14264, and Japanese Examined Patent Publication No. 62-140358, and a process in which the surface of lithium or a lithium alloy is treated or covered with a carbon material is disclosed in Japanese Unexamined Patent Publication No. 59-157973. Indeed, it carbon black or other materials having a specific surface area are dispersed in the negative electrode the substantial effective area of the negative electrode is increased, but, the activity is increased and occurrence of side reactions with the electrolyte and the like becomes frequent. Practically, carbon black or the others cannot be used as the electrode active substance, and if only the electrode surface is covered with a carbon material, an effect of improving the charging-discharging efficiency or maintaining the performance cannot be obtained.
Attempts to use, as the electrode active substance, a composite body comprising a conductive polymer having a conjugated double bond in the main chain instead of the above-mentioned carbon material, and an alkali metal or alkali metal alloy are disclosed in Japanese Unexamined Patent Publication No. 60-26351, Japanese Unexamined Patent Publication No. 61-245474, and Japanese Unexamined Patent Publication No. 62-140358. Where lithium or a lithium alloy is used, as described above with respect to the composite material with the carbon material, the surface area of the electrode can be increased but side reactions are undesirably promoted.
In connection with the above-mentioned attempts, only when the alkali metal alloy is a sodium alloy, i.e., only when the sodium alloy disclosed in Japanese Unexamined Patent Publication No. 61-245474 is used, improvements are effectively obtained and it is possible to elevate the electric capacity density of the negative electrode to a practical level. This is because since the reactivity of the sodium metal alloy is milder than that of lithium, the lithium alloy or sodium, if an appropriate electrolyte is selected, almost all of the side reactions can be controlled, and is the sodium metal alloy is combined with a conductive polymer, the effective surface area can be increased without a promotion of side reactions.
The conductive polymer per se has an inherent low electrical conductivity and the conductive polymer is electrically insulating form the viewpoint of the battery reaction unless it is doped with an alkali metal or another dopant. Accordingly, if an undoped conductive polymer is merely dispersed in the electrode, only a liquid-retaining effect is attained in the electrode. This doping is naturally effected when the composite body is immersed in the electrolyte, because of the potential difference between the conductive polymer and the alkali metal alloy, but the speed is low. Doping an undoping occur according to the potential of the negative electrode at charging and discharging of the battery, but the conductive polymer is not always uniformly doped. Therefore, the conductive polymer must be doped with an alkali metal before it is formed into the composite body. Whether doping may be carried out electrochemically or chemically, a burden is imposed on the industrial utilization thereof and this preliminary doping is not preferable from the economical viewpoint. Furthermore, as pointed out hereinbefore, if the doped conductive polymer is used for formation of the composite body, since a part of the alkali metal ion that should inherently act as the active substance for charging and discharging is caught by the conductive polymer, an excess of the alkali metal ion becomes necessary, and thus the capacity density of the electrode is reduced. The doping quantity of the conductive polymer changes according to the electrode potential, and if the doping quantity is small, the electric conductivity is reduced. Accordingly, when the doped conductive polymer is used for the electrode of the battery, the range of the effective electrode potential by the doped conductive polymer as the constituent of the composite material is narrowed and the range of the utilizable battery voltage is much restricted.
Fabrication of a high-performance battery having a high energy density and a long cycle life is not attainable only by improvements of the constituents of the negative electrode. Note, these improvements must be combined with an appropriate positive electrode material and an appropriate electrolyte. For example, where TiS.sub.2 or MoS.sub.2 is used as the positive electrode material, the obtainable battery voltage is 2.5 V at highest, whether the lithium type active substance or the sodium type active substance may be used for the negative electrode, and a high-performance battery having a high energy density cannot be provided.
The use of sodium-cobalt oxide as the positive electrode material was proposed, for example, in the Journal of Solid State Chemistry, 6, pages 532-537 (1973), and an excellent positive electrode was provided. But, at that time, a negative electrode material effective for the reaction of inserting and releasing sodium ions had not been developed and an electrolyte for a battery was not investigated. Therefore, this positive electrode was evaluated only by using a propylene carbonate solvent not suitable for the sodium type negative electrode, and a battery comprising this positive electrode has not been developed.
Where propylene carbonate or ethylene carbonate is used alone or in the form of a mixture with another solvent as the solvent of the electrolyte, if the negative electrode is composed of metallic lithium having a smooth surface, the surface area is small, the amount of the reaction product with the solvent is small and the reaction product can act as a protecting film. However, this solvent reacts violently with an alkali metal negative electrode having a specific surface area increased for forming a high-capacity type battery, and the solvent cannot be used in this case. Therefore, an appropriate solvent must be developed. But, in the lithium metal composite negative electrode, solvent having a sufficient electric conductivity and stable to the positive electrode has not been developed.
In short, the foregoing, problems are unsolved even now, and an alkali metal secondary battery having a high energy density and a long cycle life, and capable of being industrially produced at a low cost, has not been developed.